In situ measurement of mesopelagic particle sinking rates and the control of carbon transfer to the ocean interior during the Vertical Flux in the Global Ocean (VERTIGO) voyages in the North Pacific

• Published in: Deep Sea Research Part II: Topical Studies in Oceanography
• Main Authors: Trull, T, Bray, SG, Buesseler, KO, Lamborg, CH, Manganini, S, Moy, C, Valdes, J
• Format: Article in Journal/Newspaper
• Language: English
• Published: Pergamon 2008
• Subjects: Earth Sciences; Oceanography; Chemical Oceanography; Pacific; Subarctic
• Online Access: http://www.sciencedirect.com; https://doi.org/10.1016/j.dsr2.2008.04.021; http://ecite.utas.edu.au/54605

Among the parameters affecting carbon transfer to the ocean interior, particle sinking rates vary threeorders of magnitude and thus more than primary production, f-ratios, or particle carbon contents [e.g.,Boyd, P.W., Trull, T.W., 2006. Understanding the export of marine biogenic particles: is there consensus?Progress in Oceanography 4, 276312, doi:10.1016/j.pocean.2006.10.007]. Very few data have beenobtained from the mesopelagic zone where the majority of carbon remineralization occurs and theattenuation of the sinking flux is determined. Here, we report sinking rates from 300m depth for thesubtropical (station ALOHA, June 2004) and subarctic (station K2, July 2005) North Pacific Ocean,obtained from short (6.5 day) deployments of an indented rotating sphere (IRS) sediment trap operatingas an in situ settling column [Peterson, M.L.,Wakeham, S.G., Lee, C., Askea, M.A., Miquel, J.C., 2005. Noveltechniques for collection of sinking particles in the ocean and determining their settling rates.Limnology and Oceanography Methods 3, 520532] to separate the flux into 11 sinking-rate fractionsranging from 4820 to 42md 1 that are collected by a carousel for further analysis.Functioning of the IRS trap was tested using a novel programming sequence to check that allparticles have cleared the settling column prior to the next delivery of particles by the 6-hourly rotationcycle of the IRS. There was some evidence (from the flux distribution among the cups andphotomicroscopy of the collected particles) that very slow-sinking particles may have been under--collected because they were unable to penetrate the brine-filled collection cups, but good evidence forappropriate collection of fast-settling fractions.Approximately 50% of the particulate organic carbon (POC) flux was sinking at greater than100md 1 at both stations. At ALOHA, more than 15% of the POC flux sank at 4820md 1, but low fluxesmake this uncertain, and precluded resolution of particles sinking slower than 137md 1. At K2, lessthan 1% of the POC flux sank at 4820md 1, but a large fraction ( 1545%) of the flux was contributedby other fast-sinking classes (410 and 205md 1). PIC and BSi minerals were not present in higherproportions in the faster sinking fractions, but the observations were too limited to rule out a ballastingcontribution to the control of sinking rates. Photographic evidence for a wide range of particle typeswithin individual sinking-rate fractions suggests that biological processes that set the porosity andshape of particles are also important and may mask the role of minerals.Comparing the spectrum of sinking rates observed at K2 with the power-law profile of fluxattenuation with depth obtained from other VERTIGO sediment traps deployed at multiple depths[Buesseler, K.O., Lamborg, C.H., Boyd, P.W., Lam, P.J., Trull, T.W., Bidigare, R.R., Bishop, J.K.B., Casciotti,K.L., Dehairs, F., Elskens, M., Honda, M., Karl, D.M., Siegel, D., Silver, M., Steinberg, D., Valdes, J., VanMooy, B., Wilson, S.E., 2007b. Revisiting carbon flux through the Oceans twilight zone. Science316(5824), 567570, doi:10.1126/science.1137959] emphasizes the importance of particle transformationswithin the mesopelagic zone in the control of carbon transport to the ocean interior.